Photopolymerizable compositions are used to form a photoresist (or resist) layer on a substrate, such as copper-clad glass epoxy, to allow subsequent selective processing of the substrate, such as selective etching or electroplating. Photopolymerizable compositions are also used to form a permanent solder mask layer on an imaged substrate to protect the underlying circuit lines from solder exposure during the later step of soldering in printed circuit board (PCB) manufacture. The use of photoresists and solder masks for manufacture of printed circuit boards (PCBs) is described in Photoresist--Materials and Processes, by William S. DeForest, McGraw-Hill, 1975, in Printed Circuits Handbook, edited by Clyde F. Coombs, Jr., McGraw-Hill, 1988 (3rd Edition), and in Handbook of Printed Circuit Manufacturing by Raymond H. Clark, Van Nostrand Reinhold, 1985. Photopolymerizable resist materials are known, for example, from U.S. Pat. No. 3,469,982 which describes a film resist with a sandwich structure in the form of a photopolymerizable layer between a cover sheet and a temporary support. Photoresist and solder mask products that have substantially all of the solvent for coating removed are known as dry films. The use of photopolymerizable proofing films for off-press proofing in the printing industry is described in Principles of Color Proofing, by Michael H. Bruno, GAMA Communications (Salem, N.H.), 1986. The use of flexographic photopolymer films for use in production of flexographic printing plates is described in Flexography--Principles and Practices, Fourth Edition, Foundation of Flexographic Technical Association, (Ronkonkoma, N.Y.), 1992.
As the technology level of printed circuit boards increases towards finer lines/spaces, the demands placed on the photoresist materials used to produce such boards becomes greater. This in turn places a greater burden on the resist formulator to accommodate improved performance, without a decrease in production level or an increase in manufacturing cost of the photoresist product. Similiar considerations apply for use of photopolymerizable compositions in proofing and flexographic printing.
For use in the manufacture of printed circuit boards (PCBs) having fine lines/spaces, it is critical that the photoresist used possess properties such that, upon imaging, the exposed and developed photoresist affords resist patterns having good sidewall geometry (i.e., sidewalls are smooth, planar and form an angle of 90.degree. with respect to the substrate surface, and are free of imperfections such as mousebites, gouges, foot(s), etc.). Photoresists that afford good sidewall geometry will perform better in printed circuit board manufacture than otherwise comparable photoresists that do not afford good sidewall geometry. Photoresists that do afford good sidewall geometry normally exhibit higher resolution, are useful in production of finer line/space PCBs, and give fewer defects and higher yields in printed circuit board manufacture than do otherwise comparable photoresists that do not afford good sidewall geometry.
Photopolymerizable compositions somewhat related to those of this invention are disclosed in 1) Japanese Kokai Patent Application No. HEI 7[1995]-333846 and in 2) Japanese Kokai Patent Application No. HEI 4[1992]-153275. The former (#1) discloses photopolymerizable compositions containing a binder system containing two polymeric binder components and does not include any compositions with a single binder component. The latter (#2) discloses electrodeposition compositions that are made dispersible in aqueous media by neutralization or partial neutralization of the acid functionality of the binder with a basic compound.
While compositions containing two or more binder components, such as those disclosed in Japanese Kokai Patent Application No. HEI 7[1995]-333846, can be useful as photoresists for some applications, they often do have limitations associated with their use, particularly in high resolution applications required to make state of the art PCBs having fine lines and spaces. Particularly, when the two or more binders differ substantially in their properties, such as molecular weight, acid number, etc., then the two or more binders are not fully compatible with each other or the others, and the binders in this event can and often do partially or substantially separate into separate phase domains when the binders are present in the photopolymerizable compositions(s). When such phase separation does occur, it can result in development defects, such as for example, certain domains being more easily washed out during development processing and resulting in an imaged resist having defective sidewalls where there are gouges or mousebites present in the resist sidewall. Such imperfections are highly detrimental toward achieving fine lines/spaces in PCB manufacture with resists having such multiple binders.
There is a continuing need for effective photopolymerizable compostitions having inherent propensities for affording imaged resists having good sidewall geometry and low defect levels, particularly for applications involving generation of imaged photopolymer patterns (e.g., imaged resists) having fine lines/spaces.